As is well known, the degree of integration of semiconductor integrated circuits has been being remarkably enhanced in recent years. Attendant on this trend, shifting to shorter wavelengths of exposure light sources in lithographic process in the production of semiconductor devices has progressed. At present, the main stream is photolithography in which excimer lasers ranging from KrF excimer laser (248.3 nm) to ArF excimer laser (193.4 nm) are used. The introduction of immersion technique for realizing a higher NA is considered to progress from now on, in order to achieve further refining or miniaturization of semiconductor devices. In addition, enhancement of the output of the ArF excimer laser used as a light source is considered to progress, for the purpose of enhancing the throughput during the production of semiconductor devices.
Attendant on the tendency toward light sources with shorter wavelengths and lenses with higher NA values, such optical parts used in exposure apparatuses as lens, window, prism, and synthetic quartz glass for photomask are required to be more enhanced in precision. Especially, in regard of the ArF excimer laser, there are a multiplicity of important requirements such as high ultraviolet (UV) transmittance, high uniformity of transmission properties, stability and uniformity of transmittance at the time of irradiation with an excimer laser. Further, depending on the adoption of polarized illumination, reduction of birefringence in plane is also required.
As for the transmittance to UV rays, in the ArF excimer laser, for example, the transmittance to rays of a wavelength of 193.4 nm is the most important. In the case of synthetic quartz glass, its transmittance to rays in the wavelength range around 193.4 nm is lowered according to the content of impurities. Typical examples of the impurities are alkali metals such as Na, and metallic elements such as Cu and Fe. In the production of synthetic quartz glass, a silane or silicone having an extremely high purity may be used as a raw material, whereby the concentration of metallic impurities in the resulting synthetic quartz glass can be lowered to such a level (<1 ppb) as to be non-detectable even if measured by use of a high-sensitivity detector. In the cases of Na and Cu, however, their diffusion coefficients in synthetic quartz glass are comparatively high, so that in many cases Na and Cu may be externally diffused and mixed into the product during a heat treatment. Therefore, these treatments have to be carried out with special care for reducing the possibility of such contamination.
Other than the impurities, structural defects in synthetic quartz glass are also known to exert influences on the transmittance. Famous examples of the defects are deficiency of oxygen and excess of oxygen in the Si—O—Si structure constituting the synthetic quartz glass, such as oxygen deficiency type defects (Si—Si; with absorption at 245 nm) and oxygen excess type defects (Si—O—O—Si; with absorption at 177 nm). In the case of synthetic quartz glass products for UV rays, those glass products in which these defects are present at such a level as to be measurable by general spectrometry have to be excluded at least from the beginning.
The structural defects in quartz glass, such as oxygen deficiency type defects and oxygen excess type defects, will not only lower the transmittance at wavelengths of 300 nm or below but also cause a lowering in the stability of the quartz glass at the time of irradiation with an excimer laser. Particularly, the damage in the quartz glass by ArF excimer laser is said to be five times greater than that by KrF excimer laser; therefore, this is a very important factor when the quartz glass is used for lens in exposure apparatus or the like.
In the case where synthetic quartz glass is irradiated with ArF excimer laser, it causes a phenomenon in which oxygen deficiency type defects are cleaved due to the very intense energy of the laser light to generate paramagnetic defects called E′ center (E prime center), which cause absorption at 215 nm. This leads to a lowering in transmittance of the synthetic quartz glass at a wavelength of 193.4 nm. Furthermore, the E′ center generation are known to cause rearrangement of a network structure in the quartz glass, resulting in a phenomenon called “laser compaction” in which density and refractive index are raised.
It is known that for improving the stability of synthetic quartz glass to irradiation with a laser, it is extremely effective to reduce the above-mentioned intrinsic defects of the synthetic quartz glass and, simultaneously, to set the concentration of hydrogen molecules in the synthetic quartz glass at a certain level or above. Besides, the fact that hydrogen molecules in synthetic quartz glass hinder the damage to the synthetic quartz glass due to irradiation with an excimer laser has been well known and under ardent researches, since it was shown in JP-A H01-212247 (Patent Document 1).
On the other hand, in recent years, a phenomenon called “laser rarefaction” in which the density and refractive index of quartz glass are lowered, contrary to the laser compaction, has come to be a problem. The laser rarefaction is considered to arise from OH groups present in quartz glass. Therefore, it is said to be preferable to use synthetic quartz glass having a low OH group concentration, as the quartz glass applied to ArF excimer laser lithography, particularly, to immersion lithography in which a polarized light source is used.
Synthetic quartz glass with a low OH group concentration is generally produced by a method called soot process or indirect process, in which silica particulates obtained by flame hydrolysis of a silica raw material are deposited and grown, followed by heating under a reduced pressure (vacuum) to vitrify the deposited and grown material into transparent glass.
Synthetic quartz glasses for ArF excimer laser which are produced by the soot process have also been disclosed, for example, in JP-A 2003-221245 (Patent Document 2) and JP-A 2005-179088 (Patent Document 3), wherein the synthetic quartz glass have a low OH group concentration, and do not have such structural defects as oxygen deficiency type defects, and in which the generation of compaction and rarefaction is restrained.
JP-A H09-059034 (Patent Document 4) shows that quartz glass having a low OH group concentration and having few oxygen deficiency type defects is obtained by a method in which a porous quartz glass body is synthesized from a high-purity silicon compound through vapor-phase chemical reactions, the porous quartz glass body is heat treated in an oxygen-containing atmosphere, and thereafter the glass body is vitrified into transparent glass in a vacuum.
JP-A 2005-067913 (Patent Document 5) shows that, for restraining the generation of oxygen deficiency type defects and oxygen excess type defects when synthesizing silica particulates by the soot process, it is necessary to set the molar ratio of H2 to O2 (H2/O2) in a range from 2.0 to 3.0. It has been found, however, that in the production of synthetic quartz glass having a low OH group concentration by the soot process (VAD process), synthetic quartz glass having few structural defects cannot necessarily be obtained even when the method of the above-mentioned patent document is used.